Method of producing optoelectronic devices with control of light propagation by proton bombardment

ABSTRACT

In an optoelectronic device, light is restrained from propagating in one or both confining layers on either side of an active layer by forming one or more photon absorbing barriers in one, or both, confining layers. A photon absorbing barrier can be formed by proton bombardment of a confining layer, by producing a protrusion from the substrate into the adjacent confining layer, or by producing a protrusion from a capping layer into the other confining layer, or by combinations of these. Spaced apart barriers can define a device, or sections of a multisectioned device, for example a monolithic light emitting diode and modulator.

This application is a divisional of application Ser. No. 765,900 filedFeb. 7, 1977, now U.S. Pat. No. 4,115,150 which in turn is a divisionalof Ser. No. 694,333 filed 6-9-1976, now U.S. Pat. No. 4,080,617.

This invention relates to optoelectronic devices and the control oflight propagation therein, particularly to at least reduce the emissionof photons from, or into, other than the desired areas of a device.

In optoelectronic devices, such as light emitting diodes (LED's),lasers, modulators and detectors, there is often a need to accuratelydefine the active region of these devices, that is the region at whichlight emission or light absorption takes place. For example, inheterostructure GaAs/GaAlAs devices, a fraction of the light which iseither generated directly in the active layer (LED's or lasers) orcoupled into the active region (modulators or detectors) will escapefrom the active or guiding layer since the confinement will not beperfect. Such unguided light may exit through the GaAlAs confininglayers adjacent to the guiding layer and trigger undesired opticalresponse of subsequent optical elements.

The present invention provides a way of at least reducing the effects ofimperfect confinement by providing a barrier, or barriers to theunguided light in the confining layers. This invention will be readilyunderstood by the following description of certain embodiments, by wayof example, in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagrammatic cross-section through a device illustrating thebasic concept of the invention;

FIG. 2 is a curve illustrating the light transmission for differentparts of the device in FIG. 1;

FIG. 3 is a curve illustrating the ratios of light transmission throughbombarded and nonbombarded regions of a device as in FIG. 1, fordifferent wavelengths;

FIG. 4 is a diagrammatic cross-section through an integratedLED-modulator device incorporating one form of the invention;

FIG. 5 illustrates the improvement in modulator extinction ratios, usingthe invention;

FIGS. 6 to 9 illustrate steps in the production of an LEDemitter-modulator structure, incorporating one form of the invention,FIGS. 8 and 9 being cross-sections on the lines VIII--VIII and IX--IX ofFIGS. 6 and 7 respectively;

FIG. 10 is a diagrammatic cross-section of the finished structure, as onthe line X--X of FIG. 7;

FIG. 11 is a diagrammatic cross-section of a finished structureincorporating another form of the invention;

FIG. 12 is a diagrammatic cross-section of a structure, similar to thatof FIG. 11, but incorporating the present invention in a further form;

FIGS. 13 and 14 illustrate diagrammatic cross-sections through twodevices which have well-defined active or guiding layers into whichoptically absorbing barriers are introduced by proton bombardment orcrystal growth techniques respectively.

The present invention provides a barrier, or barriers which areincorporated into the confining layer, or layers of doubleheterostructure devices to prevent unguided light in those confininglayers from exiting through the side facets. This provides a variety ofadvantages for LED's, lasers, modulators, and detectors. One advantageis that the only light which exits from these devices comes from theactive or guiding layer. This will essentially eliminate undesiredoptical responses generated by stray light. A second advantage is thatthe geometry of the active region is well-defined so that emitting areascan be made comparable in size to the cores of optical fibers whichmight be attached to the end faces. A third advantage occurs inelectroabsorption or phase modulators in which the light which escapesinto and propages along the confining layers will reduce the modulationdepth. For example, integrated LED emitter-modulator structures canachieve up to 20dB extinction ratios via the process ofelectroabsorption. However these high extinction ratios are onlyachieved by limiting the area of the detector so that only light whichexits from the active, or guiding layer is recorded by the detector. Ifthe light which propagates outside the active, or guiding layer is alsodetected, then the extinction ratio is significantly reduced.

In a typical double heterostructure device, the guiding or active layerconsists of Ga_(1-y) Al_(y) As material with y≲0.1; such a layer has anoptical absorption edge in the range 800-870 nm and will guide photonswith wavelengths longer than the absorption edge value. It is proposedthat one way of overcoming light spill of these photons into and out ofthe Ga_(1-x) Al_(x) As confining layers is by introducing opticallyabsorbing regions into the confining layers by the method of protonbombardment. Thus photons from the guiding layer must be absorbed in aconfining layer of completely different material (typically Ga₀.7 Al₀.3As) where the band edge is near 680 nm. This is a completely differentsituation to that in which proton bombardment of GaAs providesabsorption for wavelengths close to the GaAs absorption edge. The basicvalidity of the above proposal has been established using the deviceillustrated in FIG. 1. A 5 μM thick Ga₀.7 Al₀.3 As layer was first grownon an n-GaAs substrate. Approximately one half of the area of this layerwas bombarded at 390 keV, 3 × 10¹⁵ cm⁻² ; the other half was shieldedfrom the beam. The crystal was then glued to a glass slide with atransparent photoresist and the whole of the n-type substrate wasremoved by using a selective etch (H₂ O₂ +NH₄ OH, pH = 8.70). After anetch time of about 60 minutes, only the 5 μm thick layer remained. InFIG. 1 the glass slide is indicated at 10, the photoresist layer at 11and the 5 μm thick Ga₀.7 Al₀.3 As layer is indicated at 12. The protonbombarded area is indicated at 13. Light from a monochromatic source wasthen passed through both the bombarded and nonbombarded regions of thecrystal, as indicated by arrows X, and detected by a cooledphoto-multipler.

FIG. 2 illustrates a typical variation in the transmitted lightvariation across a crystal for a fixed wavelength of 750 nm. Theundulations are due to surface roughness of the etched surface but thelocation of the boundary 14 between bombarded and nonbombarded regionsis easily identified. The two intensities of light are indicated on FIG.1, and as an average on FIG. 2, as τ_(b) and τ_(o) for bombarded andnonbombarded regions respectively.

FIG. 3 illustrates how the ratio of light transmission through theunbombarded and bombarded regions of a crystal (i.e. τ_(o) /τ_(b))varies as a function of wavelength. As will be seen, the ratio variesfrom about 1.2 for wavelengths of 725 nm to about 0.45 for wavelengthsof 900 nm.

As an example of a device employing the invention, an integratedLED-modulator structure is illustrated in FIG. 4. The structureillustrated is a double heterostructure comprising a GaAs substrate 20,a first Ga_(1-x) Al_(x) As (x ≃ 0.3) confining layer 21, an active GaAslayer 22, a second Ga_(1-x) Al_(x) As (x ≃ 0.3) confining layer 23 andan optional capping layer 24. A masking layer 25 is formed on thecapping layer 24 and proton bombardment forms regions 26 of high opticalabsorption in the second confining layer 23 (and in the capping layer 24although this is incidental). The conductivity type of the layers canvary provided there is the correct relationship. Thus the substrate isn-type, the first confining layer and the active layer are n-type whilethe second confining layer (and capping layer) are p-type. If thesubstrate is p-type, the first confining layer and active layer are alsop-type and the second confining layer (and capping layer) are n-type.

Hole 27 is then etched through the substrate 20. A suitable etch is asreferred to previously, for removal of the substrate in the preparationof the device of FIG. 1. Conveniently the etch is selective for GaAs,stopping at the first confining layer 21, the bottoms of the hole 27being at the boundary between substrate 20 and confining layer 21. Afurther proton bombardment is carried out from the substrate side of thestructure to form a region 30 of high optical absorption at the bottomof the hole as well as along the periphery of the hole. The LED emittersection is at 28 and is energized by an appropriate potential or biasapplied to the capping layer 24 in the emitter section and to thecontact on the substrate 20. The modulator section 29 modulates thelight emission from the active layer 22 in the emitter section, again bysuitable potentials applied to the capping layer 24 and substrate 20.

The photons labelled B and C pass into the confining layers 21 and 23.Photons B will be absorbed by the proton damaged regions 26 and 27. Thephotons C will be absorbed to some extent by the proton damage at theperiphery of the hole 26, indicated at 30. Complete absorption in thesubstrate can be assured if the n-type active layer 22 contains a smallamount of Al which will shift the photon energy to values beyond theabsorption edge of substrate 20. A detector is indicated at 31.

The possible improvement gains are illustrated by the curves in FIG. 5.The curves illustrate extinction ratio versus effective detector widthat the modulator exit face. The highest extinction ratios are obtainedwhen the effective detector width is narrower than the thickness of theguiding layer 22. When the effective width is greater than the guidinglayer 22, so that light from the confining layers 21 and 23 is alsoincluded, the extinction ratio is reduced by 8-10 dB. By preventing thepropagation of light rays through the confining layers of the modulator,the size of the effective detector width is not so critical. A widereffective detector width can be used and still obtain high extinctionratios. The extinction ratio will be improved by 8-10 dB relative to thesame wider effective detector width without photon absorption. Curves 33and 34 illustrate extinction ratio versus effective detector width fortwo conventional modulator devices operated at negative biases of 24volts and 18 volts, respectively. Curves 33a and 34a illustrate theimprovements achieved by introducing optical absorption into theconfining layers by proton bombardment.

FIGS. 6 to 9 illustrate two steps in producing a high-speed highextinction ratio LED emitter-modulator structure, and FIG. 10 is across-section through the structure-on the line X--X of FIG. 7. Thestructure illustrated is a double heterostructure, as in FIG. 4, with asubstrate 35, first confining layer 36, active or guiding layer 37, asecond confining layer 38 and optional capping layer 39. Theconductivity type of substrate 35 and layers 36, 37, 38 and 39 aspreviously described in relation to FIG. 4. High speed operation isobtained by limiting the junction capacity with a first protonbombardment. A metal stripe 40 is produced on the capping layer 39 andthe structure bombarded. The bombardment alters the layers 39, 38 and37, also part of the layer 36, as seen in FIG. 8. Narrow gaps 42 arethen etched into the metal stripe and a second proton bombardment whichalters only layers 39 and 38 forms electrical isolation between sectionsand ensures optical absorption in the top confining layer 38. Thestructure is then as in FIG. 9. Finally holes 43 are etched through thesubstrate 35 to the first confining layer 36 and a third protonbombardment is performed into these holes from the substrate side, toform regions 44 which provide optical isolations (absorption) in thelower, or first, confining layer. The mask layer for etching the holes43 and masking from the third bombardment is indicated at 45. A certainamount of bombardment damage also occurs on the sides of the holes 43 at46. For both second and third bombardments the proton beam energy isaccurately controlled to ensure that protons penetrate only to a minimumextent into the active or guiding layer 37. In FIG. 10, the emittersection is the central one third portion between the bombarded regions42. The structure illustrated in FIG. 10 has one LED emitter section 46with modulator sections 47 positioned on either side. These devices aremost conveniently made by fabricating a large number of sections on acommon substrate and then dividing along the dashed line 48 (FIG. 10).

In relation to FIGS. 6 to 10, if high modulator speed is not critical,the first proton bombardment can be eliminated. The only bombardmentrequired is that which creates regions 42, (now extending in stripes allacross the crystal) to provide electrical (and optical) isolationbetween sections and prevent propagation of leakage photons in thesecond confining layer 38. It is also possible to provide an alternativeoptical isolation structure for the first confining layer. In FIG. 11 aphoton absorbing region is formed by initial profiling of a substrate.As illustrated in FIG. 11 a substrate 50 is masked and etched on onesurface to form upstanding ribs or ridges 51. The first confining layer52 is then formed followed by formation of the active or guiding layer53. The thickness of layer 52 can be controlled for careful crystalgrowth such that the gap between the top surface of the ribs 51 andactive layer 53 is small. The second confining layer 54 is formedfollowed by capping layer 55. A masking layer 56 is formed on thecapping layer 55 and isolation regions 57 are formed by protonbombardment through layers 54 and 55 down to the upper surface of theactive or guiding layer 53. The proton bombardment regions 57 preventpropagation of photons B along the upper or second confining layer 54into the modulator section 60, from the emitter section 61, while theribs 51 absorb the photons B propagating in the lower or first confininglayer 52. The photons C are absorbed in the substrate. To ensuresubstantially complete absorption of unwanted protons the active layer53 contains some Al, having the form n-Ga_(1-y) Al_(y) As with y ≃ 0.1.The photons A will be the only light to emit from the emitter 61 andpropagate through the modulator 60.

As an alternative to the proton bombarded regions 57 which were used inFIG. 11 to provide optical absorption, some devices can effectivelyutilize profiling plus crystal growth techniques to provide the requiredoptical absorption in both confining layers. This is illustrated in FIG.12 using the same references as in FIG. 11 where applicable. In thiscase, upstanding ridges 51 provide optical absorption in the firstconfining layer while inverted ridges 62 provide optical absorption inthe second confining layer. In such an arrangement the capping layer 55is not optional and would have properties the same as substrate 50, thatis, be essentially GaAs (with little or no Al content). This wouldensure absorption of photons B provided the guiding or active layer 53is of GaAlAs, for example Ga₀.9 Al₀.1 As.

In addition to the integrated emitter-modulator devices discussedpreviously, the inventions are applicable to many discreteoptoelectronic devices such as LED's, laser, modulators and detectors.Diagrammatic cross-sections are shown in FIGS. 13 and 14 for the casesof optical barriers introduced by the techniques of proton bombardmentand crystal growth, respectively. In each case a well-defined active orguiding layer is defined which provides those advantages discussedpreviously. In these devices, the structure comprises a substrate 63with a p- or n-type active layer 64 and confining layers 65 and 66 oneither side of the active layer. A capping layer 67 is on top ofconfining layer 66. Substrate and first confining layer are typicallyn-type while second confining layer and capping layer are of p-type.Precise definition of the active layer is provided at the exit facets at68 by proton bombardment in FIG. 13 and at 69 by crystal growth in FIG.14.

What is claimed is:
 1. A method of producing an optoelectronic device,comprising:etching one surface of a substrate of semiconductor materialto produce at least one protrusion extending normal to said surface ofsaid substrate; forming a first confining layer on said substrate, saidlayer extending over said protrusion, said layer and said substrate thesame conductivity type; forming an active layer on said first confininglayer and a second confining layer on said active layer, said secondconfining layer being of opposite conductivity type as said firstconfining layer and said active layer being of the same conductivitytype as one of said confining layers; said protrusion forming a photonabsorbing barrier.
 2. A method as claimed in claim 1, including etchingsaid second confining layer to form at least one aperture extending tosaid active layer; andforming a capping layer of semiconductive materialon said second confining layer of the same conductivity type as saidsecond confining layer, said capping layer extending into said apertureand forming a photon absorbing barrier.
 3. A method as claimed in claim1, including forming a plurality of spaced apart protrusions on saidsubstrate.
 4. A method as claimed in claim 1, including protonbombarding at least one region in said first confining layer to form aphoton absorbing barrier in said first confining layer.